4. Combine 2/3 cup milk and the next 4 ingredients (through black pepper) in a small saucepan over medium-high heat, stirring with a whisk until smooth. Cook for 2 minutes or until mixture is thick and bubbly, stirring constantly. Spoon mixture into a large bowl, and let stand for 10 minutes. Stir in spinach, cheese, and egg yolks.

5. Combine egg whites and cream of tartar in a large bowl, and let stand at room temperature for 15 minutes. Beat with a mixer at high speed until medium peaks form (do not overbeat). Gently stir one-fourth of egg whites into spinach mixture, and gently fold in the remaining egg whites. Gently spoon mixture into prepared dishes. Sharply tap dishes 2 or 3 times on counter to level. Place dishes on preheated baking sheet; return baking sheet to 425° oven. Immediately reduce oven temperature to 350°; bake soufflés at 350° for 21 minutes or until puffy and golden brown. Serve immediately.